1. Field of the Invention
The present invention relates to the field of current Feedback instrumentation amplifiers.
2. Prior Art
The notation Gm as used in this disclosure refers to a transconductance amplifier, used in plurality to construct and instrumentation amplifier.
Instrumentation amplifiers (IA) are frequently used in sensor interfacing and current sensing. Current feedback instrumentation amplifiers (CFIA) gain favor because they can combine low noise and low supply power with an intrinsic high CMRR (common mode rejection ratio). The combination of low noise and low supply power is only true if their differential input stages are not being degenerated. However, non-degenerated input stages exhibit a mismatch which over temperature is not better than 2% untrimmed and 0.2% trimmed. The inaccuracy of these amplifiers directly depends on the gain mismatch of the two input stages. For most Instrumentation Amplifier applications, this inaccuracy is too high.
FIG. 1 is a block diagram of a prior art chopper current feedback instrumentation amplifier. The first input stage Gm21 connects to the input through chopper Ch21, while the second input stage Gm22 connects to a feedback attenuator R1 and R2 at the output through chopper Ch22. The ratio of the gains of the input stages divided by the attenuation of the output attenuator determines the overall gain.
The voltage-to-current functions of the input Gm's isolate the common mode input voltage from the common mode output voltage. This results in an intrinsic high common mode rejection ratio without the need of a resistor bridge that needs to be trimmed.
The input stages will have offset voltages Vos21 and Vos22. When choppers Ch1 and Ch21 and Ch22 are placed around the input stages, the average offset will be greatly reduced and the CMMR will be greatly enlarged (the offset Vos1 of Gm1 is divided down by the gain of Gm21 and is of no substantial consequence). However, a ripple will result in the output voltage. A Ripple Reduction Loop (RRL) can be applied to kill the ripple. For this purpose, two capacitors Cs31 and Cs32 sense the ripple voltage at the Instrumentation Amplifier output. Their current is synchronously demodulated by a chopper Ch3, and the demodulated current is integrated on the capacitors C41 and C42 connected around the Gm4, or alternatively, one capacitor connected across the differential output of a current buffer CB4. The output voltage of the integrator represents the amplified offset. This offset is fed back to the output of the input stages through the correction amplifier Gm3. If the loop gain is high, the offset and ripple nearly completely vanish. The offset and ripple can both be reduced by a factor 100 to 1000.